Fix simplified sample floating dock updates

[wieslawsoltes]

Summary

• replace non-observable List<IDockable> layout collections in DockSimplifiedSample with the factory's observable collections
• extract the sample layout into a focused, testable DockFactory
• add a headless regression that floats Tool 1, observes the layout update, redocks it beside the document, and verifies the floating window is removed without losing either dockable

Root cause

Docking operations mutated the sample model, but its plain List<T> collections did not notify Avalonia. The visual tree therefore remained stale while the model removed and reinserted dockables.

Validation

• dotnet test tests/Dock.Avalonia.HeadlessTests/Dock.Avalonia.HeadlessTests.csproj -c Release --no-restore (614 passed)
• dotnet build Dock.slnx -c Release --no-restore (0 errors)

Closes #1044